linux-kernel - Re: [PATCH] xen: introduce xen_vring_use

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Date:   Sat, 11 Jul 2020 14:44:34 -0400
From:   "Michael S. Tsirkin" <mst@...hat.com>
To:     Stefano Stabellini <sstabellini@...nel.org>
Cc:     jgross@...e.com, Peng Fan <peng.fan@....com>,
        konrad.wilk@...cle.com, jasowang@...hat.com, x86@...nel.org,
        linux-kernel@...r.kernel.org,
        virtualization@...ts.linux-foundation.org,
        Christoph Hellwig <hch@...radead.org>,
        iommu@...ts.linux-foundation.org, linux-imx@....com,
        xen-devel@...ts.xenproject.org, boris.ostrovsky@...cle.com,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH] xen: introduce xen_vring_use_dma

On Fri, Jul 10, 2020 at 10:23:22AM -0700, Stefano Stabellini wrote:
> Sorry for the late reply -- a couple of conferences kept me busy.
> 
> 
> On Wed, 1 Jul 2020, Michael S. Tsirkin wrote:
> > On Wed, Jul 01, 2020 at 10:34:53AM -0700, Stefano Stabellini wrote:
> > > Would you be in favor of a more flexible check along the lines of the
> > > one proposed in the patch that started this thread:
> > > 
> > >     if (xen_vring_use_dma())
> > >             return true;
> > > 
> > > 
> > > xen_vring_use_dma would be implemented so that it returns true when
> > > xen_swiotlb is required and false otherwise.
> > 
> > Just to stress - with a patch like this virtio can *still* use DMA API
> > if PLATFORM_ACCESS is set. So if DMA API is broken on some platforms
> > as you seem to be saying, you guys should fix it before doing something
> > like this..
> 
> Yes, DMA API is broken with some interfaces (specifically: rpmesg and
> trusty), but for them PLATFORM_ACCESS is never set. That is why the
> errors weren't reported before. Xen special case aside, there is no
> problem under normal circumstances.

So why not fix DMA API? Then this patch is not needed.


> 
> If you are OK with this patch (after a little bit of clean-up), Peng,
> are you OK with sending an update or do you want me to?